Test for API breaking changes in CI

[xFrednet]

Currently, the CI only checks that the marker_api crate can be compiled on stable. It's up to the reviewer and programmer to ensure that the API doesn't break. This is fine right now, as the API is under active development and still changes frequently. However, the goal of Marker is to create a stable API.

The rust ecosystem has tools to prevent accidental public API changes, like cargo-semver-checks. Once v0.1.0 or a higher version has been released, I think it would be good to utilize such a tool.

This issue is blocked for now until v0.1.0 or a later point, when we've defined what stability means for marker and ii API breakage will be allowed under specific circumstances.

[xFrednet]

I've done a bit of testing on this, but nothing extensive. One thing I noticed, is that by default, it compares the API to the version from crates.io. This means that once the API is broken on master, every PR would fail this check. Instead, the semver check should check if the changes of the PR add new breaking changes, in comparison to master.

During initial development, v0.* API breakage is allowed. During this time, I want to have this check, to ensure that we don't break the API unintentionally and that these changes are clearly tracked in our changelog.